The action reportedly came within a 36-hour period over the weekend in the country's restive North Caucasus region.

The area is home to a simmering Islamist insurgency, with some fighters there having sworn allegiance to Islamic State. A spokesman for anti-terrorist committee said security operations were carried out to "stop terror attacks" and also gave more details on one of the confrontations.

Russia's national anti-terrorism committee spokesman, Andrei Chatsky: "Over the course of several hours talks were held with an armed group. The militants refused to lay down their arms and opened fire at law enforcement officers. In the firefight that followed six militants were killed."

Rights activists and Kremlin critics accuse Russian security services of heavy-handed tactics in the North Caucasus. They say Russia targets not only militants but also dissenters in operations which sow discord among locals.